This invention relates to a cylinder block of an internal combustion engine.
Typically, the cylinder block of the engine has a deck surface and a cylinder head is fastened over the deck surface using so-called “head bolts”, with a head gasket (hereinafter referred to simply as “gasket”) placed between the cylinder block and the cylinder head. See for example JP 2000-240502 A, paragraphs 0019–0026, FIGS. 1–5.
In this instance, the cylinder block and the cylinder head provided in the engine are die-cast components formed of a light alloy for weight reduction. The cylinder block, gasket and cylinder head of the engine each have bolt holes provided at predetermined positions corresponding to one another, and head bolts made of steel are inserted into the bolt holes of the cylinder head, gasket and cylinder block, and then screwed in the bolt holes provided in the cylinder block, thereby fastening the cylinder block and the cylinder head together.
However, the cylinder block as disclosed in JP 2000-240502 A is made of a light alloy such as an aluminum (Al) alloy or a magnesium (Mg) alloy, which is greater in coefficient of thermal expansion than the head bolts made of steel. Therefore, when the engine becomes hot during operation, the bolt holes of the cylinder block tend to expand in their axial directions more than the head bolts, and thus the axial fastening force of the head bolts becomes greater.
Accordingly, the guaranteed strength against the axial fastening force of the head bolts should be determined with consideration given to the extent to which the axial fastening force becomes greater. In cases where a greater axial fastening force should be applied to the head bolts, head bolts with a higher grade of strength need to be selected. Consequently, head bolts having a larger diameter enough to exhibit a strength required to endure the axial fastening force corresponding to the selected higher grade of strength for the head bolts should be provided, which would disadvantageously restrict flexibility in designing an engine layout, or offer some other problems.
When the cylinder block made of an aluminum alloy having a greater coefficient of thermal expansion expands and contracts due to change in temperature resulting from operation of the engine, the axial fastening force of the head bolts would change considerably, which would disadvantageously impair the sealing capability of the gasket at the deck surface.
In view of the aforementioned disadvantages, it is appreciated that the axial fastening force exerted to the head bolts should preferably fall invariably within a specific range irrespective of change in temperature, i.e., regardless of whether the joint of the head bolts is under high temperature conditions or under low temperature conditions. In other words, there is a need to provide a cylinder block in which the increase in axial fastening force of the head bolts can be checked or prevented.
Disclosed in JP 2002-224816 A (paragraphs 0023, 0044; FIG. 1) is another example of a cylinder block made of an aluminum alloy for an internal combustion engine, in which reinforcements are provided on a cylinder bore (hereinafter referred to simply as “bore”) to improve resistance to abrasion and to reduce resistance to the sliding action of a piston.
Typically, a cylinder liner made of such reinforcements or a cast iron of good quality, etc. is provided in a bore of the cylinder block for consideration of the resistance to abrasion and the slidability required for the bore.
However, the reinforcements of the cylinder block as disclosed in JP 2002-224816 A are made by integrally embedding into the cylinder block a metal porous body composed of a stainless steel such as Fe, Cr and Ni, and are thus heavier in mass in comparison with metal materials, such as aluminum alloys, making up the cylinder block, which would place the cylinder block at disadvantages in achieving weight and size reduction.
The cylinder block as disclosed in JP 2002-224816 A is adapted to improve the slidability of the bore, but when increase in output of an engine of the same type is desired, the engine could not help undergoing major design changes in order to attend to the associated increase in heat load and combustion pressure; this would entail the problems as follows.
(1) As the combustion pressure increases, a bearing stress placed on a gasket disposed between the cylinder head and the cylinder block should be raised to a level enough to seal an interface between the cylinder block and the cylinder head and to block combustion gases from escaping. Accordingly, in this cylinder block of which the rigidity around the bore is so low that the efficiency in application of the axial fastening force to the bolt-fastened portion of the cylinder block disposed around the bore is low, the axial fastening force of the head bolts would disadvantageously be required to be increased more to compensate the diminished axial fastening force.
(2) The increase in the bearing stress placed on the gasket would lower a buckling strength of this cylinder block at a surface on which the gasket is fastened. Therefore, a great likelihood of buckling at the surface on which the gasket is fastened would disadvantageously make it impossible to place a sufficient bearing stress on the gasket.
(3) The increase in heat load placed on a portion of this cylinder block around the bore would disadvantageously lower the heat dissipating characteristics of the portion around the bore because reinforcing of the portion around the bore could lower the heat conduction characteristics thereof.
In the above and other instances of the cylinder block for which an aluminum alloy is used and provided around the bore, disadvantages that would be entailed are as follows.
(1) Upon startup of the engine at a low temperature, a piston that is different in temperature rise characteristic from this cylinder block is raised in temperature more swiftly than the cylinder block, which would require that a clearance determined with consideration given to the coefficient of thermal expansion of the piston be provided between the cylinder block and the piston.
(2) The higher the coefficient of thermal expansion of the bore is, the more the bore expands after the engine is warmed up, which would make the clearance between the cylinder block and the piston larger, causing the piston to unsteadily sway more violently, and thus increasing noises and vibrations.
Further, the loosened fit of a piston ring of the piston would disadvantageously result in an escape of oil into a combustion chamber, which would enormously increase the amount of wasted oil, and/or an escape of combustion gas into a clunk chamber, which would greatly deteriorate the quality of oil.
In view of the circumstances, it would be desirable to provide a cylinder block having a high thermal conductivity, high rigidity and low coefficient of expansion of portions around the bore, without sacrificing the slidability of the piston inside the bore, as well as achieving weight and size reduction, and higher output.
Illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an illustrative, non-limiting embodiment of the present invention may not overcome any of the problems described above.